This application relates to methods for synthesizing Salacinol, its stereoisomers, and analogues thereof potentially useful as glycosidase inhibitors.
In treatment of non-insulin dependent diabetes (NIDD) management of blood glucose levels is critical. One strategy for treating NIDD is to delay digestion of ingested carbohydrates, thereby lowering post-prandial blood glucose concentration. This can be achieved by administering drugs which inhibit the activity of enzymes, such as glucosidases, which mediate the hydrolysis of complex starches to oligosaccharides in the small intestine. For example, carbohydrate analogues, such as acarbose, reversibly inhibit the function of pancreatic xcex1-amylase and membrane-bound intestinal xcex1-glucoside hydrolase enzymes. In patients suffering from Type II diabetes, such enzyme inhibition results in delayed glucose absorption into the blood and a smoothing or lowering of postprandial hyperglycemia, resulting in improved glycemic control.
Some naturally-occurring glucosidase inhibitors have been isolated from Salacia reticulate, a plant native to submontane forests in Sri Lanka and parts of India (known as xe2x80x9cKotala himbutuxe2x80x9d in Singhalese). Salacia reticulata is a woody climbing plant which has been used in the Ayurvedic system of Indian medicine in the treatment of diabetes. Traditionally, Ayurvedic medicine advised that a person suffering from diabetes should drink water left overnight in a mug carved from Kotala himbutu wood. In an article published in 1997, Yoshikawa et al. reported the isolation of the compound Salacinol from a water-soluble fraction derived from the dried roots and stems of Salacia reticulate.1 Yoshikawa et al. determined the structure of Salacinol, shown below, and demonstrated its efficacy as an xcex1-glucosidase inhibitor. 
Yoshikawa et al. later reported the isolation from the roots and stems of Salacia reticulate of Kotalanol which was also shown to be effective as an xcex1-glucosidase inhibitor.2 Like Salicinol, Kotalanol contains a thiosugar sulfonium ion and an internal sulfate providing the counterion: 
Kotalanol has been found to show more potent inhibitory activity against sucrase than Salicinol and acarbose.2 
The exact mechanism of action of Salacinol and other glucosidase inhibitors has not yet been elucidated. Some known glycosidase inhibitors, such as the indolizidine alkaloids castanospermine and swainsonine, are known to carry a positive charge at physiological pH. 
It is believed that the mechanism of action of some known inhibitors may be at least partially explained by the establishment of stabilizing electrostatic interactions between the inhibitor and the enzyme active site carboxylate residues. It is postulated that the compounds of the present invention, which comprise postively charged sulfonium, ammonium, and selenonium ions, could function in a similar manner. It is also possible that Salacinol and other compounds of the same class may act by alteration of a transport mechanism across the intestinal wall rather than by directly binding to glucosidase enzymes.
Salacinol and Kotalanol may potentially have fewer long-term side effects than other existing oral antidiabetic agents. For example, oral administration of acarbose in the treatment of Type II diabetes results in undesirable gastrointestinal side effects in some patients, most notably increased flatulence, diarrhoea and abdominal pain. As mentioned above, Salacinol has been used as a therapy for diabetes in the Ayurvedic system of traditional medicine for many years with no notable side effects reported. Further, recent animal studies have shown that the oral ingestion of an extractive from a Salacia reticulate trunk at a dose of 5,000 mg/kg had no serious acute toxicity or mutagenicity in rats.3 
The Salacia reticulate plant is, however, in relatively small supply and is not readily available outside of Sri Lanka and India. Accordingly, it would be desirable if Salicinol, Kotalanol and analogues thereof could be produced synthetically.
Carbohydrate processing inhibitors have also been shown to be effective in the treatment of some non-diabetic disorders, such as cancer. While normal cells display characteristic oligosaccharide structures, tumor cells display very complex structures that are usually found in embryonic tissues. It is believed that these complex structures provide signal stimuli for rapid proliferation and metastasis of tumor cells. A possible strategy for therapeutic use of glucosidase inhibitors is to take advantage of the differential rates of normal vs cancer cell growth to inhibit assembly of complex oligosaccharide structures. For example, the indolizidine alkaloid swainsonine, an inhibitor of Golgi xcex1-mannosidase II reportedly reduces tumor cell metastasis, enhances cellular immune responses, and reduces tumor cell growth in mice.4 Swainsonine treatment has led to significant reduction of tumor mass in human patients with advanced malignancies, and is a promising drug therapy for patients suffering from breast, liver, lung and other malignancies.5,6 
The compounds of the present invention may also find application in the treatment of Alzheimer""s disease due to their stable, internal salt structure. Alzheimer""s is characterized by plaque formation in the brain caused by aggregation of a peptide, xcex2-amyloid, into fibrils. This is toxic to neuronal cells. One can inhibit this aggregation by using detergent-like molecules. It is believed that the compounds of the present invention, which are amphipathic, may demonstrate this activity.
The need has therefore arisen for a new class of glycosidase inhibitors which may be synthesized in high yields from readily available starting materials and which have potential use as therapeutics.
In accordance with the invention, a compound selected from the group consisting of non-naturally occurring compounds represented by the general formula (I), including stereoisomers and pharmaceutically acceptable salts thereof is disclosed, 
where X is selected from the group consisting of S, Se, and NH. Such compounds include stereoisomers of Salicinol. The target compounds have a stable, internal salt structure comprising heteroatom cation X and a sulfate anion; the substituents may vary without departing from the invention. Preferably, R1, R2, R3, R4 and R5 are the same or different and are selected from the group consisting of H, OH, SH, NH2, halogens and constituents of compounds selected from the group consisting of cyclopropanes, epoxides, aziridines and episulfides; and R6 is selected from the group consisting of H and optionally substituted straight chain, branched, or cyclic, saturated or unsaturated hydrocarbon radicals, such as alkyl, alkenyl, alkynyl, aryl, and alkoxy substituents containing any suitable functionality.
Processes for the production of compounds of the general formula (I) are also disclosed comprising reacting a cyclic sulfate having the general formula (II) with a 5-membered ring sugar having the general formula 
where X is selected from the group consisting of S, Se, and NH; R1 and R2 are selected from the group consisting of H and a protecting group; R3 is selected from the group consisting of H and optionally substituted straight chain, branched, or cyclic, saturated or unsaturated hydrocarbon radicals and their protected derivatives; and R4, R5 and R6 are the same or different and are selected from the group consisting of H, OH, SH, NH2, halogens and constituents of compounds selected from the group consisting of cyclopropanes, epoxides, aziridines and episulfides and their protected derivatives. Preferably the cyclic sulfate is a 2,4-di-O-protected-D-or L-erythritol-1,3-cyclic sulfate, such as 2,4-O-Benzylidene-D-or L-erythritol-1,3-cyclic sulfate (i.e. R1 and R2 comprise a benzylidene protecting group); R3 is H or a protected polyhydroxylated alkyl chain; and R4, R5 and R6 are selected from the group consisting of OH and a protected OH group, such as OCH2C6H5. The synthetic processes comprise the step of opening the cyclic sulfate (II) by nucleophilic attack of the heteroatom X on the sugar (III).
In an alternative embodiment of the invention, the cyclic sulfate (II) may be reacted with a 6-membered ring sugar having the general formula (XI) to yield a compound having the general formula (XII): 
where X is selected from the group consisting of S, Se and NH. In this embodiment R1, R2, R3, R4, R5 and R6 are the same or different and are selected from the group consisting of H, OH, SH, NH2, halogens and constituents of compounds selected from the group consisting of cyclopropanes, epoxides, aziridines and episulfides and R7 is selected from the group consisting of H and optionally substituted straight chain, branched, or cyclic, saturated or unsaturated hydrocarbon radicals. Preferably R1, R2and R3 are as described above in respect of compound (II) and R4, R5, R6 and R7 are selected from the group consisting of H, OH, SH, NH2, halogens and constituents of compounds selected from the group consisting of cyclopropanes, epoxides, aziridines and episulfides and their protected derivatives.
The application also relates to pharmaceutical compositions comprising an effective amount of a compound according to formula (I) or (XII) together with a pharmaceutically acceptable carrier and to methods of treating carbohydrate metabolic disorders, such as non-insulin dependent diabetes, or different forms of cancer or Alzheimer""s disease by administering to a subject in need of such treatment an effective amount of such compounds.
Salacinol is a naturally occurring compound which may be extracted from the roots and stems of Salacia reticulata, a plant native to Sri Lanka and India. This application relates to synthetic routes for preparing Salacinol (1), and its nitrogen (2) and selenium (3) analogues shown below. 
This application also relates to synthetic routes for preparing the stereoisomers of compounds (1) to (3). Such analogues and stereoisomers (including stereoisomers of Salacinol) comprise a new class of compounds which are not naturally occurring and may find use as glycosidase inhibitors.
Scheme 1(a) below, shows the general synthetic scheme developed by the inventors for arriving at the target compounds. To synthesize different stereoisomers of Salacinol and its nitrogen and selenium analogues (A)-(C), 5-membered-ring sugars are reacted with sulfate-containing compounds in accordance with the invention (in Scheme 1(a) the letters (A), (B), and (C) represent all stereoisomers of Salacinol and its nitrogen and selenium analogues (1), (2) and (3) respectively). The inventors followed a disconnection approach for determining the preferred synthetic route. A reasonable disconnection is one that gives the 5-membered-ring sugars (D) since they can be synthesized easily from readily available carbohydrate precursors. Nucleophilic substitution at C1 of the sulfate fragment (E) can then yield the target molecules (Scheme 1(a)). A potential problem with this approach is that the leaving group (L) might act later as a base to abstract the acidic hydrogens of the sulfonium salt7 and produce unwanted products. Therefore, the cyclic sulfate (F) may be used instead of (E) to obviate the problems associated with leaving group (L). Compound (G) may similarly be used as a cyclic sulfate reagent and is a protected version of (F). 
Scheme 1(b) below shows generally the coupling reactions for producing the target compounds (A)-(C). 
Route 1 of Scheme 1(b) shows the general strategy of reacting a cyclic sulfate with a 5-membered ring sugar to produce an intermediate compound, which may include benzyl or other protecting groups. As described in further detail below, the intermediate compound is then deprotected to yield the target compounds. The inventor s have determined that Route 2 of Scheme 1(b), a possible side reaction, does not occur.
Cyclic sulfates and 5-membered-ring sugars were prepared in accordance with the synthetic schemes described below. As will be apparent to a person skilled in the art, other equivalent schemes for producing the reagents of the invention could be substituted.
Cyclic sulfates were prepared in analogous fashion to the ethylidene acetal.8 The cyclic sulfate (7) was synthesized in 4 steps starting from D-glucose (Scheme 2). 2,4-O-Benzylidene-D-erythrithol (5) was synthesized from D-glucose in two steps,9,10 and then treated with thionyl chloride to yield the cyclic sulfite (6) which was oxidized to the cyclic sulfate (7) as described by Calvo-Flores et al.8
The enantiomer (10) was also synthesized using the same route but starting from L-glucose (Scheme 3). 
In order to synthesize one of the 5-membered-ring sugars (D, X=S), 1,4-anhydro-3-O-benzyl-4-thio-D-arabinitol (11), was synthesized in 9 steps starting from D-glucose (Scheme 4).11 Benzylation of the compound (11), using benzyl bromide in DMF yielded 1,4-anhydro-2,3,5-tri-O-benzyl-4-thio-D-arabinitol (12) in 90% yield. Compound (11) was debenzylated to give 1,4-anhydro-4-thio-D-arabinitol (13) in 97% yield using a Birch reduction. 
The L-isomer, 1,4-anhydro-2,3,5-tri-O-benzyl-4-thio-L-arabinitol (14) was synthesized in 5 steps starting from D-xylose (Scheme 5).12 
1,4-Di-O-methanesulfonyl-2,3,5-tri-O-benzyl-D-xylitol (15) is also a key intermediate for the synthesis of the aza and selena sugars (16) and (17). 1,4-Dideoxy-1,4-imino-L-arabinitol (16)13 was synthesized in 7 steps starting from D-xylose (Scheme 5). The enantiomer (19)13 was synthesized in an analogous way starting from L-xylose (Scheme 6). Compound (19) was also synthesized in 10 steps starting from D-xylose.13 1,4-Anhydro-2,3,5-tri-O-benzyl-4-seleno-D-arabinitol (20) was synthesized in 5 steps starting from L-xylose (Scheme 6). To synthesize compound (20), Na2Se was made in-situ by treatment of selenium metal with sodium in liquid ammonia. 
Scheme 6(a) below shows a more generalized scheme for synthesizing compound (20) using other possible protecting groups (R=COR, CH2C6H4xe2x80x94OMep). 
The target compounds (1)-(3) were prepared by opening of the cyclic sulfates by nucleophilic attack of the heteroatoms on the 5-membered rings (Scheme 1(b) above). The heteroatom gives rise to a positively charged cation and the cyclic sulfate gives rise to a negatively charged counterion. This internal salt structure may explain the stability of the target compounds toward decomposition by further nucleophilic attack.
Salacinol (1) was synthesized by nucleophilic substitution of the protected thio-arabinitol (12) with the cyclic sulfate (10) (1.2 equiv) in dry acetone containing K2CO3, to give the protected intermediate compound (21) in 33% yield. Hydrogenolysis of the benzyl and benzylidene groups in AcOH:H2O, 4:1 afforded Salacinol (1) in 67% yield (Scheme 7). 
The same procedure was used to prepare intermediate compound (22) in 79% yield from the enantiomeric cyclic sulfate (7). Deprotection as before gave compound (23) in 59% yield (Scheme 8). Compound (23) is a diastereomer of Salacinol (1). 
Compound (24) was prepared in 40% yield from (7) and the enantiomeric thio-ether (14) (Scheme 9). Deprotection in 80% yield gave the enantiomer of Salacinol (25). 
To reduce the number of synthetic steps, the inventors attempted the coupling reactions with the deprotected thio-arabinitols. Thus, the partially deprotected compound (11) was reacted with the cyclic sulfate (10) in acetone, to give compound (26) in 32% yield. Deprotection yielded Salacinol (1) in 36% yield (Scheme 10). 
The fully-deprotected thio-arabinitol (13) was not soluble in acetone and the reaction in methanol produced several products.
The seleno-analogue intermediate (27) (R=CH2C6H5) was made starting from the seleno-arabinitol (20) (R=CH2C6H5) and the cyclic sulfate (10) in excellent yield 86% (Scheme 11), but NMR spectroscopy showed the presence of two isomers in a ratio of 7:1 that differed in stereochemistry at the stereogenic selenium center. The isomers were separable by analytical HPLC. The inventors have assigned the name xe2x80x9cBlintolxe2x80x9d to the new selenium analogue (3). 
The seleno-analogue intermediate (28) (R=CH2C6H5) was made starting from the seleno-arabinitol (20) (R=CH2C6H5) and the cyclic sulfate (7) in excellent yield 97% (Scheme 12); a mixture of two isomers in a ratio of 3:1 that differed in stereochemistry at the stereogenic selenium center was obtained. The isomers were separable by analytical HPLC. 
Compound (29) is a diastereomer of Blintol (3).
The nitrogen analogue intermediate (30) was made by the reaction of the deprotected imino-arabinitol (19) with the cyclic sulfate (10) in a good yield 72% (Scheme 13). Compound (19) was not soluble in acetone so the reaction was performed in dry methanol. A side product (19%) which was identified to be the product of methanolysis of the cyclic sulfate was obtained. The inventors have assigned the name xe2x80x9cGhavamiolxe2x80x9d to the new nitrogen analogue (2). Compound (30) was deprotected to give Ghavamiol (2) in 64% yield. 
The enantiomer intermediate (31) was made by the reaction of the deprotected imino-arabinitol (16) with the cyclic sulfate (7) in a good yield 72% (Scheme 14). A side product (21%) which was identified to be the product of methanolysis of the cyclic sulfate was obtained. Compound (31) was deprotected to give compound (32) in 77% yield. Compound (32) is the enantiomer of Ghavamiol (2). 
In an alternative embodiment of the invention, target compounds having potential application as glycosidase inhibitors may be synthesized in the manner described above using 6-membered rather than 5-membered ring heterocycles as reagents. As in the embodiments described above, the cyclic sulfate (described above) is opened in the coupling reaction due to nucleophilic attack of the heteroatoms (i.e. X=S, Se, N) on the ring sugars. As will be apparent to a person skilled in the art, the general formulas for the 6-membered sugar reagent and resulting target compound are as shown below. 
The 6-membered ring target compound shares the same internal salt structure as the 5-membered ring embodiment. The substituent groups may vary as described above without departing from the invention.